Build or buy? That question tops the list when systems designers decide how to supply dc power at the growing number of voltage and current levels present in virtually every type of electronic system. As supply voltage levels drop and currents rise, the task of powering microprocessors, ASICs, DSPs, logic, and analog elements in these systems is becoming more difficult.
Because power design has long been considered as much art as science, the first inclination may be to leave it to the experts—that is, buy power modules or bricks from established power-supply providers. But semiconductor manufacturers have developed control and driver ICs and support tools that take some of the mystery out of supply design. Now such design is accessible to engineers who want to build at least some of their system's power supplies but have little practical design experience.
The thinking in today's system power design is to use a distributed-bus architecture based on a chain of dc-dc converters that take a dc voltage generated from the ac line and break it down to the individual voltages needed to supply the system's digital elements. The first dc-dc converter in this chain is a bus converter that takes a high dc voltage, say 48 V, converted from the ac line, and distributes it to secondary supplies called point-of-load (POL) converters (Fig. 1). Each POL delivers a required voltage or voltages and current to specific elements of the system, such as microprocessors, memory, ASICs, and DSPs. With supply voltages falling below 3.3 V (2.5, 1.8, and 1.5 V are common in systems) and current demand rising (50 to 100 A or greater) in high-speed systems, it could take a large number of POLs to supply the system's loads.
A distributed-bus architecture contains two types of dc-dc converters: The bus converter must be an isolated supply, while the POLs can be non-isolated. Isolation means that the bus converter must have a transformer to separate the ac power line from the load, both for personnel and equipment safety. A discrete isolated dc-dc converter design can be quite complicated, while a non-isolated design is much simpler. This is where the build or buy decision comes into play.
"Most people buy an isolated supply, a brick or a version of one, to go from 48 V down to bus voltage," says Steve Goacher, marketing manager for Texas Instruments. "Isolated converters are complicated designs that require expertise to meet requirements such as FCC noise specifications."
Isolated designs are more complex, agrees Donald Ashley, strategic marketing manager at National Semiconductor. "The transformer is the difficult part of the design. It takes a bit of mathematics to solve the design equations, but it depends on the complexity of the design. If a novice did a few non-isolated designs, he might figure out an isolated one because there's so much knowledge and help available about non-isolated buck regulator designs," he says.
Since power-supply design can be daunting to the uninitiated, National offers an evaluation board that lets users play with a fully functional converter before venturing out on their own. The LM5030 evaluation board is designed around the LM5030 high-voltage PWM controller, mounted on a board that measures 2.4 by 2.4 by 0.5 in. It can be used for push-pull or bridge topologies, has an input voltage range of 36 to 75 V, and delivers a 3.3-V output at up to 10 A.
On the board, two alternating outputs drive two N-channel MOSFETs. These feed the two halves of the power transformer primary. A feedback path using an optocoupler (the LM3411) provides isolation and drives the COMP pin on the LM5030. This voltage controls the pulse width to the output MOSFETs. A number of built-in supervisory features, such as dual-mode current limit, soft start, sync capability, and thermal shutdown, further ease the novice designer's task.
Expertise to design an isolated converter is one part of the build or buy decision. The other is the number of systems a company plans on manufacturing. If volumes are low, say less than 100 systems per year, the consensus among semiconductor marketing managers is that it's better to buy an isolated module or brick from a cost standpoint and treat it as a component rather than attempt to design one. Yet as volumes increase, so does the rationale for doing a discrete design. Goacher believes that systems houses may start out with a brick but switch to a discrete design to get a cost reduction as volumes ramp up.
Putting it in dollar terms, Madhu Rayabhari, power management marketing director at Fairchild Semiconductor, says, "It's a tradeoff. You have to weigh the costs of investing the time to design and debug an isolated converter and save somewhere between $15 to $80 on the bill of materials versus purchasing an off-the-shelf power supply."
When it comes to build-buy decisions about the simpler-to-design non-isolated POL converters, the picture is much clearer. Semiconductor manufacturers have come up with a variety of controller and driver ICs that remove much of the complexity involved in power-supply design. The goal is to eliminate the expertise factor in supply design and allow relatively inexperienced designers to successfully implement their own designs using a building-block approach. But realizing that hardware alone can't ensure a working design, chip makers offer a comprehensive support structure that includes software design tools, reference designs, evaluation boards, schematics, component values, and access to experienced designers on their staffs.